Hard disk drives can fail, which is the reason that many users back up their data, or at least should back up their data, on a regular basis. However, when a hard disk drive fails, any data that was written to the hard disk drive since the time of the last backup is likely to be lost, given contemporary computing models having a single hard disk drive in the system.
Solutions such as RAID (redundant array of independent disks, which may include disk mirroring concepts) help protect against such loss of data. However, such solutions are expensive, can be inefficient and can degrade performance, and are thus not desirable to many computer users.